The present invention relates quite generally to the operation of LEDs, these being understood to be both inorganic LEDs and organic LEDs (OLEDs). In particular, the invention relates to an emergency light function which is used when an external power supply for the LEDs is interrupted.
In principle, it is already known to supply an LED path, which can have one or more series-connected LEDs, with electrical power starting from a constant-current source. It is also known to use a PWM modulation for carrying out dimming so that in the turn-on periods of a PWM pulse train, the said constant current control is carried out. During the dimming, the duty ratio of the PWM signal is thus changed then.
To provide the supply voltage of the constant-current source, it is possible to use, for example, an actively clocked PFC (Power Factor Correction) circuit.
Finally, other requirements must also be considered when operating LEDs, for example, that DC isolation is usually demanded between the LED path and the supply voltage of the PFC, typically an alternating line voltage.
The result of the above is that for operating an LED path properly and advantageously, several circuit blocks must be functionally present (PFC, DC isolation, constant-current source etc.). This may lead to relatively complex circuits.
For operating an LED lighting device, the latter is usually connected to the existing power supply system, the abovementioned electronic components for power matching being coordinated therewith. Although this enables an LED lighting device to be used without limitation in the area in which the power supply system is available, the fundamental functionality of the LED lighting device is thus always dependent on the power supply system. In the event of a failure thereof, therefore, lighting is also impossible. Furthermore, in this case the problem arises that the LED lighting device can no longer be controlled. Modern lighting systems have a communication bus by means of which a wide variety of information can be exchanged between the individual lamps and a central control unit. Such a communication is likewise precluded in the event of a power failure.